My iPhone 4 restarts itself exactly every 3 min, whether I use it or not (screen off). More precisely, it's respringing its springboard; I don't need to unlock my SIM card after each respring. Seems like a software issue as it restarts at such precise intervals, but a firmware restore has not helped.
Worth mentioning is that it's running iOS 4.2.1 jailbroken, and the problem started after a color swap I did myself. Everything else (all the hardware parts including cameras, antennas, Wi-Fi, etc.) works just fine—which makes it hard to believe that this problem would occur only because of opening the iPhone up. Because of all this, I can't rely on my warranty, so returning the phone is not an alternative.
Best Answer
Did your iPhone was jailbroken before the color swap?
If so, I'm afraid you broke something inside the phone. Something the OS is trying to access and since it's broken, respring the springboard (maybe that's a bug, but since you're not supposed to fiddle with the inside of the phone, it's not something you could use to replace it).
As a last resort, you could try to reopen it and see if there's anything that looks broken.